Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate, typically expressed in units of beats per minute (bpm). They can occur in either chamber of the heart (i.e., ventricles or atria) or both. Examples of tachyarrhythmias include sinus tachycardia, ventricular tachycardia, ventricular fibrillation (VF), atrial tachycardia, and atrial fibrillation (AF). Tachyarrhythmias can be due to abnormal excitation by normal pacemaker tissue, an ectopic excitatory focus, or a re-entry phenomenon. Tachycardia occurs when the heart contracts relatively normally but at a rapid rate, while fibrillation occurs when the chamber depolarizes in a chaotic fashion with abnormal depolarization waveforms as reflected by an EKG.
An electrical defibrillation shock applied to a heart chamber can be used to terminate most tachyarrhythmias by depolarizing excitable myocardium and rendering it refractory. Implantable cardioverter/defibrillators (ICDs) provide this kind of therapy by delivering a shock pulse to the heart when fibrillation is detected by the device. ICDs can be designed to treat either atrial or ventricular tachyarrhythmias, or both, and may also incorporate cardiac pacing functionality. The most dangerous tachyarrhythmias are ventricular tachycardia and ventricular fibrillation, and ICDs have most commonly been applied in the treatment of those conditions. Another type of electrical therapy for tachycardia is antitachycardia pacing (ATP). In ATP, the heart is competitively paced with one or more pacing pulses in an effort to interrupt the reentrant circuit causing the tachycardia. Modem ICD's usually have ATP capability so that ATP therapy is delivered to the heart when a tachycardia is detected, while a shock pulse is delivered when fibrillation occurs.
ICDs are also capable of detecting atrial tachyarrhythmias, such as atrial fibrillation and atrial flutter, and delivering a shock pulse to the atria in order to terminate the arrhythmia. Although not immediately life-threatening, it is important to treat atrial fibrillation for several reasons. First, atrial fibrillation is associated with a loss of atrio-ventricular synchrony which can be hemodynamically compromising and cause such symptoms as dyspnea, fatigue, vertigo, and angina. Atrial fibrillation can also predispose to strokes resulting from emboli forming in the left atrium. Although drug therapy and/or in-hospital cardioversion are acceptable treatment modalities for atrial fibrillation, ICDs configured to treat atrial fibrillation offer a number of advantages to certain patients, including convenience and greater efficacy.
As described above, an implantable device may deliver appropriate therapy to terminate certain detected arrhythmias. Such therapies are not invariably successful, however, and, even when they are, may require repeated application until the arrhythmia is finally terminated. Defibrillation shocks also subject the patient to some discomfort. It would be more beneficial if an implantable device could detect when a pre-arrhythmic condition exists and deliver electro-stimulatory therapy in a manner that prevents the arrhythmia from occurring in the first instance.